Re: [RFC PATCH] perf_counter: Add support for pinned and exclusive counter groups


* Paul Mackerras <paulus@xxxxxxxxx> wrote:

Ingo Molnar writes:

(btw., the percpu allocation code seems to have bitrotten a bit - the
logic around perf_reserved_percpu looks wrong and somewhat complicated.)

Yes, it has. I don't think it was ever fixed to apply only to hardware
counters when software counters were added.

Side question - were you intending to make the various software counters
able to act as interrupting counters? That will mean e.g. that anything
that increments current->maj_flt or current->min_flt (i.e.
do_page_fault, __get_user_pages) will need to check if that causes any
counter to overflow, which could be a bit invasive.

Yes, eventually my intention was to bring all the sw counters up to that
level, and allow system sampling via sw counter overflows. It's an
arguably powerful concept.

I first wanted to see where they all go though, and how many of them we
want, and how nuanced we want to make them.

Initially we can sample via __builtin_return_address(0) addresses, or
pt_regs(current)->rip or so. Later on we could use save_stack_trace()
perhaps, for a more vectored sample.

It would result in some rather cool kerneltop output: we could see a
profile of which functions generate pagefaults, or which places generate
context-switches, which places migrate a lot, etc.

Again, which restrictions users/developers are more willing to live
with will be shown in actual usage of these facilities.

Indeed.

So my approach was always to have a good guess about what the best usage
pattern is, but also to not be rigid and ignore/exclude other usecases.
The main principle of this subsystem is to be maximally useful in
performance analysis, via modern and flexible abstractions. Pinning and
exclusivity fits into that just fine - they are mechanism to improve the
quality of the statistics.

Ingo
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